As illustrated in FIG. 1, generally, the entire service area is divided into a plurality of cells, and one base station apparatus is installed for each cell in a mobile communication system. Then, the respective cellular station apparatuses perform wireless communication with their own base station apparatuses, to which they belong.
In the case of FIG. 1, since cellular station apparatus 31 and 32 belong to cell 11, they perform wireless communication with base station apparatus 21 installed in cell 11. Similarly, since cellular station apparatus 33 belongs to cell 12, it performs wireless communication with base station apparatus 22 installed in cell 12.
Here, respective cellular station apparatuses transmit uplink signals and receive downlink signals using the channel allocated by their base station apparatuses. A plurality of channel allocation methods of a mobile communication system using this cellular system has been previously proposed.
One of the channel allocation methods is described in Toshihito Kanai: “Autonomous Reuse Partitioning Dynamic Channel Allocation Method in Micro Cell Mobile Communication System (ARP),” The Technical Report of IEICE, RCS91-32 (1991). With this Autonomous Reuse Partitioning (ARP) system, a channel is selected with the same order of priority in all cells, and among the selected channels, the one with a CIR (Carrier to Interference Ratio) exceeding the predetermined threshold is used sequentially.
Channel allocation in the conventional ARP system is next explained using the flowchart illustrated in FIG. 2.
First, when there is a call request at Step (hereinafter referred to as “ST”) 51, the base station apparatus measures the desired wave level of the uplink, and the cellular station apparatus measures the desired wave level of the downlink at ST52.
Then, at ST 53, the base station apparatus selects the available channel with the highest priority according to the common order of priority for all base station apparatuses. An available channel denotes an unused slot in the case of the TDMA system, and in the case of the CDMA/TDD system it denotes an unallocated slot or a slot wherein uplink/downlink to be allocated is matched and that has available code resource.
Next, at ST54, the base station apparatus measures the interference wave level of the uplink for the selected channel, and the cellular station apparatus measures the interference wave level of the downlink.
Then at ST55, the base station apparatus compares the uplink/downlink CIR of the selected channel with a predetermined threshold (the so-called channel search.) If the uplink CIR and downlink CIR exceed the threshold, the base station apparatus will allocate a call to the selected channel at ST 56. However, if either uplink CIR or downlink CIR are below the threshold, the base station apparatus will determine whether or not available channels are unchecked with the channel search (hereinafter, referred to as “non-searched channel”) at ST 57.
If non-searched channels still remain, the base station apparatus and cellular station apparatus repeat the processes following ST 53, excluding the channels completed for a channel search. Meanwhile, if non-searched channels do not remain, the base station apparatus will complete the processing as call loss at ST59.
With ARP channel allocation, it is possible to perform the so-called reuse partitioning (Halpern:“Reuse Partitioning in Cellular Systems,” Proc. of VTC '83, pp. 322–327 (1983)) in each cell in an autonomous, spreading manner, wherein the optimal cell reuse factor can be set for each channel depending on the distance from the cellular station apparatus to the base station apparatus, that is, the size of the loss of the transmission path.
Furthermore, by performing reuse partitioning and setting the optimal cell reuse factor, the system is capable of accommodating more calls as a whole.
With this conventional ARP system, it is assumed that the number of channels for an uplink and downlink is fixed at the same number and that a pair of uplink channel and downlink channel is fixed. Therefore, uplink/downlink to be allocated to each channel is common to all cells. For example, in FIG. 1 above, cellular station apparatus 33 does not transmit signals in the channel where cellular station apparatus 31 receives signals. Thus, a cellular station apparatus is not disrupted by signals transmitted from the cellular station apparatus that belongs to another cell.
However, it is expected that asymmetrical data communications, wherein the information volume of the downlink is significantly larger than that of the uplink, will be common place in the future. With this asymmetrical data communication, each channel is allocated to an uplink or downlink in a flexible manner. Namely, it is necessary to change the number of channels allocated to the uplink and downlink from the predetermined number of channels.
However, if the conventional ARP channel allocation method were applied to asymmetrical data communication and channel allocation were performed in the same order of priority for an uplink and downlink, the uplink/downlink to be allocated to each channel would likely differ depending on the cell. In this condition, a cellular station apparatus would be disrupted by signals transmitted from the cellular station apparatus belonging to another cell, and it would therefore be impossible to perform reuse partitioning.